This invention relates to the general field of acoustic imaging, and in particular, to a system which includes a liquid crystal detector cell and improved ultrasound source for enhancing image quality.
The use of ultrasonics to nondestructively and noninvasively inspect objects for internal discontinuities, irregular shapes, etc., is well known in the industrial and medical fields. In one form, ultrasonic imaging is accomplished by using electronic displays in which the ultrasonic signal is electronically detected and enhanced. In another form, the ultrasonic image is proposed to be detected and displayed using an ultrasonic transducer to insonify the object to be inspected and a liquid crystal cell to detect ultrasonic energy from the insonified object. The liquid crystal cell includes a pair of cover plates between which a layer of liquid crystal material is encapsulated. See, Greguss, U.S. Pat. No. 3,831,434 and Dion, U.S. Pat. No. 4,338,821 for examples of such proposed transducers and cells.
In such systems the ultrasonic transducer produces a coherent ultrasonic signal and the images may have defects due to the coherence of the ultrasound. With such coherent ultrasound, the defects may be produced by phase cancellations and phase reinforcements. These image defects are sometimes referred to as image artifacts. Speckle and ringing are common artifacts. "Speckle" refers to randomly positioned variations in image intensity due to phase cancellations and reinforcements. "Ringing" refers to systematic variations, due to phase cancellation and reinforcement, and which usually appear as fringes in the images.
These artifacts can be minimized by reducing ultrasonic wave coherence. Dion, U.S. Pat. No. 4,338,821, suggests reducing coherence by phase shifting through the pivoting movement of the acoustic transducer or by frequency sweeping between fixed limits which results in the superimposing of images from all frequencies swept, as suggested by previous art.
However, to applicant's knowledge, the Dion system has not produced a high-quality, substantially artifact-free image on a liquid crystal detector cell.
Another approach to producing non-coherent or incoherent ultrasonic energy was suggested in "Spatially and Temporally Varying Insonification for the Elimination of Spurious Details in Acoustic Transmission Imaging", by J. F. Havlice, et al, Acoustical Holography, Vol. 7, page 291-305 (1977), which suggested an array of twenty-five single ultrasonic sources, which are turned on independently and the final incoherent image is the summation of twenty-five single exposures. This approach is slow and requires a considerable electronic outlay.
Another approach was suggested in "Coherence and Noise in Ultrasonic Transmission Imaging" by Roder et al, Ultrasonics, November 1980, page 273-276. There a chamber was filled with water and polystyrene cylinders whose dimensions are about the wavelength of the ultrasound. Turbulent flow of the water was caused and ultrasound was directed into the chamber. Ultrasound exiting the object is to be detected by a small water tank which is in contact with a larger coupling water tank. The physical arrangement of this system has a number of disadvantages, including vibration which can render image detection more difficult, the dector can only be used in a horizontal position, convenience, the need for a Schlieren optical system for image viewing, etc. See also "Acoustic Image with Holography and Lenses", by Glenn Wade, IEEE Transaction, November 1975, page 385, et seq.
None of these non-coherent ultrasound generating systems are known to produce acceptable substantially artifact-free acoustic images in a liquid crystal cell detector.
It is therefore an object of this invention to provide a convenient source of non-coherent or incoherent ultrasonic energy for producing a substantially artifact-free image on a liquid crystal cell detector.
This and other objects will become apparent from the following description and appended claims.